As I run sudo DOWNGRADE_FROM_ALA=1 downgrade firefox
I got the version which I currently have installed (the one marked with the + sign) and furthermore the important Firefox 80 which still I don’t have available (the one which I marked with the red highlighter)
Firefox 80 will include, finally, HW acceleration on X11!
Would be the case, so, to push Firefox 80 to all branches, also for the sake of bug haunting?
Since a long time, when I want to play HTML5 videos on Firefox on Linux, I rely on MPV  which gave, also on 4K videos, a really light usage of CPU;
(otherwise, when I play videos from Firefox itself - on Linux -, I got almost 80% - 100% CPU usage); furthermore I experienced, here and there, stuttering while scrolling (please notice that: I have “layers.omtp.enabled” set to true and also “gfx.webrender.all” set to true) 
in facts, when I run the bencharmk HTML5 Fish Bowl I always got 60 FPS. But without real HW acceleration: and Firefox eats a lot of CPU usage (100%)
Heck I might argue that pacman -Uing from one of our Unstable mirrors would be preferable as well.
But still … we wait for the push … we usually have a good record on that expediency… lets see
(and who am I to judge … I left my ungoogled-chromium unbuilt for ~a week. … but its building now)
I must say although it works fine with hw acceleration it still requires a lot more CPU power compared to mpv for example. I guess I still stick to mpv (triggered by a script) to watch stuff. Especially on BAT.
For me, the CPU usage on VA-API of Firefox is almost similar to mpv (mpv on 8%-10%), when playing videos; however I still get high CPU usage when scrolling pages: it go from 20% to 80%.
Can you also check this with htop?